Real Madrid will pay the 50 million-euro release clause in installments to acquire the left-back, who will compete with Fran Garcia for a starting position.
On Friday, Benfica announced the signing of Rafael Obrador, acquired from Real Madrid for approximately five million euros in a separate agreement.
The 22-year-old Carreras had a contract with Benfica until 2029. The defender is valued at 34.3 million euros by Flashscore. Last season, he played 52 matches for the Portuguese league runners-up.
Carreras will be introduced tomorrow at noon at Ciudad Real Madrid. The president of the Spanish club, Florentino Perez, will meet with the left-back to sign his contract, which is valid for six seasons, before the press conference at the same location.